Gene interactions among genes governing protein localization, DNA replication, and the biogenesis of the cell surface will be studied using the entire range of molecular genetic methods now available to attack difficult biological problems. The organisms under study are the bacterium Salmonella typhimurium, its temperate bacteriophage P22, and the lower eukaryotic yeast Saccharomyces cerevisiae. The ideas and methods which will be used include classical genetic analysis of mutants, pseudo-reversion methods for detecting gene interactions, recombinant DNA methods, (particularly highly localized in vitro mutagenesis) and the construction of gene fusions. The value of these studies in health science is through direct understanding of these basic processes and through the development of technology which can be directly applied to specific problems. Secretion of beta-lactamase in Salmonella and invertase in yeast are being analyzed by making mutations specifically in the signal sequences that have been shown to be essential in the protein translocation process. Revertants of such mutants will be analyzed; such revertants may occur intragenically or in genes which are involved in the translocation and processing machinery. The complexes of proteins which must function together to replicate DNA will be studied in both Salmonella and yeast. Pseudoreversion will be used to find suppressor genes whose products interact with enzymes known to be involved in DNA replication. The biosynthesis of the cell wall will be studied by finding the pathway to d-alanine and the basis for O-antigen variation. These studies directly address the health problems of pathogenicity and rational antibiotic design.